A combined seismic and electromagnetic marine streamer is disclosed in GB2421800 Johnstad. The UK patent describes a system of a seismic source in the sea providing seismic signals propagating to be picked up in acoustic sensors arranged along the streamer cable, and an electromagnetic source analogically emitting electromagnetic waves for being picked up at electromagnetic sensors arranged along the same streamer cable.
Due to towing of the cable through the sea, the water turbulence formed by the streamer cable running through the water gives rise to acoustic noise. Similarly, the fact that mutually electrically connected electrodes are towed through the geomagnetic field in the sea will induce an electrical voltage to be considered as noise with respect to CSEM data acquisition. Further, the fact that the electrodes move through ion-containing water will also induce electrical noise in the measured data.
The propagating electromagnetic field will be a wave with two components, an electrical field component E and a magnetic field component H which is orthogonal to this electrical field. The magnitude of the components depends on the electromagnetic impedance Z in the medium of which the wave propagates. In marine CSEM-geophysical prospecting an EM source is usually arranged in the sea and the EM field will propagate partly downward through the sea-floor and through the rocks, and will be reflected or refracted back to the surface and may be detected by EM-sensors in the sea. The direction of propagation along a Poynting vector P=E×H may be determined by the mutual angles of the components. Thus the direction of propagation of the electromagnetic wave may be used for wavefield separation purposes in order to separate an upcoming field from the underground, which is relevant for geophysical measurements, from a downwards propagating field which may be due to an undesired air wave or a reflection from the sea surface. The relative size of the E- and H-fields as measured in the sea may also be used for calculating the electromagnetic impedance of the seafloor and the geological layers below the sea-bed.
Thus it is relevant to use an EM-sensor cable which can measure both the in-line E field and a cross-line H-field relative to the cable. At the same time it is useful to have integrated acoustic sensors in the cable in order to collect both EM and seismic measurements simultaneously. This saves time and money and provides improved correlation according to GB2421800.